Coating for a luminescent material

ABSTRACT

A metal M is chosen from the group formed by Y, Al and La, and a watery solution of a complex of M and an organic chelating agent is added to a suspension of luminescent material, which causes a layer of M 2 O 3  to be deposited on the luminescent material, which is then separated, dried, and fired.

BACKGROUND OF THE INVENTION

The invention relates to a method of coating a luminescent material witha layer of a metal oxide M₂O₃ in which a metal M is chosen from thegroup formed by Y, Al and La, in which a compound of M is deposited onthe luminescent material by means of homogeneous precipitation from awatery solution.

The invention also relates to a luminescent material obtainable by sucha method and to a discharge lamp equipped with a luminescent screencomprising such a luminescent material.

A method as mentioned in the opening paragraph is known from WO96/05265. Coated luminescent materials show a relatively high stabilityin the watery slurry that is used in the lamp manufacturing process andan improved maintenance of the light output. A drawback of the knownprocess is that many luminescent materials (for instance materialshaving a silicate host lattice) are sensitive towards hydrolysis at therelatively low pH values at which the homogeneous precipitation istaking place. Additionally it has been found that the optical propertiessuch as reflectivity and quantum efficiency of other luminescentmaterials that do not hydrolize in water are adversely effected by a lowpH value.

SUMMARY OF THE INVENTION

The invention has for its object to provide a method of coating aluminescent material with a metal oxide by means of homogeneousprecipitation from a watery solution in which hydrolysis of theluminescent material and degradation of its optical properties are to alarge extent prevented.

According to the invention, a watery suspension of the luminescentmaterial is prepared, a watery solution containing a complex of M and anorganic chelating agent is added to the watery suspension, theluminescent material is separated from the watery suspension and isdried and heated.

It has been found that the organic chelating agent prevents theprecipitation of M(OH)3. As a result the homogeneous precipitation cantake place at a relatively high value of the pH. Accordingly hydrolysisand degradation of the luminescent material are strongly suppressed.

Good results have been obtained with ethylenediamine,trispyrazolylborate, diglyme, benzoic acid, crown ethers, polyphosphatesand triazacyclononane as the organic chelating agent. More in particulargood results have been obtained with ethylenediamine tetraacetate.

Good results have also been obtained in case the pH of the waterysuspension is in the range 8-10 and the pH of the watery solution is inthe range 6.5-7.5.

Luminescent materials obtained by means of a method according to theinvention typically have a fine grainy coating consisting of a thinlayer of spherical M₂O₃ nanoparticles with a diameter smaller than 30nm. The coating can be distinguished from coatings that were applied bymeans of chemical vapor deposition (CVD) by means of scanning electronmicroscope (SEM) or transmission electron microscope (TEM) analysis. Thecoating properties can also be reflected in macroscopic phosphorfeatures such as slightly enhanced light output due to cleaner surfacewith a higher scattering compared to coatings applied by means of CVD.

A method according to the invention is particularly suitable for coatingluminescent materials having a silicate host lattice, since theseluminescent materials hydrolize very easily in a watery solution havinga relatively low pH. More in particular the method has proven to be verysuitable for the coating with La₂O₃ of BaSi₂O₅ activated with lead, (Ba,Sr)₂MgSi₂O₇ activated with lead and Zn₂SiO₄ activated with manganese.

Luminescent materials coated by means of a method according to theinvention were found to be very suitable for use in the luminescentscreen of a discharge lamp, more in particular a low pressure mercurydischarge lamp.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the first embodiment BaSi₂O₅ activated with lead (BSP) is coated withLa₂O₃. To a solution of 250 mg La₂O₃ dissolved in 50 ml demineralizedwater 125 mg of ethylenediamine tetraacetate (EDTA) is added. The pHvalue of that solution is adjusted to approximately 7 by adding Ba(OH)₂.

To another 50 ml of demineralized water Ba(OH)2 is added until the pH isapproximately 9.5. After that, 10 gram of BSP is added to that solution.The LA(EDTA) solution is added dropwise to the BSP suspension. Afteradding all of the LA(EDTA) solution to the BSP suspension the pH of thesuspension is once more adjusted to approximately 9.5 by adding Ba(OH)₂.The suspension is then stirred for two hours and the coated BSP issubsequently separated by filtration. Finally, the phosphor is washedwith alkaline water, dried at 80 C and fired 906 C for two hours. Theemission spectrum of coated BSP was substantially identical to that ofuncoated BSP (λmax=350 nm; FVMM=38 nm). Also the reflection coefficientsfor 254 nm and 350 nm radiation were almost identical (20% and 96% foruncoated BSP and 19% and 95% for coated BSP respectively). Surprisingly,however, the quantum efficiency for 254 nm radiation of the coated BSPwas found to be 6% higher than that of uncoated BSP (94% and 88%respectively).

In a second embodiment (Ba, Sr)2M9Si207 activated with lead (SMS) iscoated with La₂O₃. The coating process used is identical to the processdescribed hereabove for the coating of BSP, except that Sr(OH)₂ was usedinstead of Ba(OH)₂. The emission spectra of coated and uncoated SMS aresubstantially identical (λmax=360 nm; FVMM=60 nm). The quantumefficiency for 254 nm radiation of coated and uncoated SMS was found tobe 75%. The reflection coefficients for 254 nm and 350 nm radiation were8% and 96% for uncoating SMS and 10% and 95% for coated SMSrespectively.

In a third embodiment Zn₂SiO₄ activated with manganese (ZSM) is coatedLa₂O₃. The coating process used is idententical to the process describedhereabove for the coating of BSP, except that NaOH was used instead ofBa(OH)₂. The emission spectra of coated and uncoated ZSM aresubstantially identical (μmax=520 nm; FVMM=41 nm). The quantumefficiency of coated and uncoated ZSM was found to be 79% and 80%respectively. The reflection coefficients for 254 nm radiation were 94%and 93% for uncoated ZS coated ZSM respectively.

What is claimed is:
 1. Luminescent material coated with a layercomprising: a metal oxide, M₂O₃, in which a metal M is chosen from thegroup consisting of yttrium, aluminum and lanthanum, said layer beingsubstantially free of M(OH)₃ and comprising granular, sphericalnanoparticles of the metal oxide, said nanoparticles having a diametersmaller than 30 nm.
 2. Luminescent material according to claim 1,comprising a silicate host lattice.
 3. Luminescent material according toclaim 1, comprising BaSi₂O₅ activated with lead.
 4. Luminescent materialaccording to claim 1, comprising (Ba,Sr)₂MgSi₂O₇ activated with lead. 5.Luminescent material according to claim 1, comprising Zn₂SiO₄ activatedwith manganese.
 6. A discharge lamp comprising a luminescent screen,said screen comprising a luminescent material with a coating of a metaloxide, M₂O₃, in which a metal M is chosen from the group consisting ofyttrium, aluminum and lanthanum, the coating being substantially free ofM(OH)₃ and comprising granular, spherical nanoparticles of the metaloxide, said nanoparticles having a diameter smaller than 30 nm.
 7. Adischarge lamp according to claim 6, wherein the discharge lamp is a lowpressure mercury discharge lamp.
 8. A discharge lamp comprising aluminescent material coated with a layer to form a coated luminescentmaterial using precipitation from a watery solution containing a complexof a metal, an organic chelating agent and a suspension of saidluminescent material, wherein said coated luminescent material isconfigured to have an enhanced light output and scattering compared tocoating applied by chemical vapor deposition, wherein the coating layercomprises a metal oxide, M₂O₃, in which a metal M is chosen from thegroup consisting of yttrium, aluminum and lanthanum, and wherein thecoating layer comprises granular, spherical nanoparticles of the metaloxide, said nanoparticles having a diameter smaller than 30 nm and beingsubstantially free of M(OH)₃.
 9. A discharge lamp comprising aluminescent material coated with a layer to form a coated luminescentmaterial using precipitation from a watery solution containing a complexof a metal, an organic chelating agent and a suspension of saidluminescent material, wherein said coated luminescent material isconfigured to have an emission spectrum and a reflection coefficientwhich are substantially identical to that of the luminescent materialwithout having said layer, while having a quantum efficiency for a 254nm radiation which is higher than that of the luminescent materialwithout having said layer, wherein the coating layer comprises a metaloxide, M₂O₃, in which a metal M is chosen from the group consisting ofyttrium, aluminum and lanthanum, and wherein the coating layer comprisesgranular, spherical nanoparticles of the metal oxide, said nanoparticleshaving a diameter smaller than 30 nm and being substantially free ofM(OH)₃.